Electrochemical devices that employ non-aqueous electrolytes, as typified by lithium secondary batteries and supereapacitors, have a high energy density, and thus are widely used as a power source for portable devices such as mobile phones and notebook computers. With the trend toward higher performance portable devices, electrochemical devices tend to increase in capacity, and therefore, ensuring safety is becoming an important issue.
A currently available lithium secondary battery employs, as a separator interposed between a positive electrode and a negative electrode, for example, a polyolefin porous film having a thickness of approximately 20 to 30 μm. In order to secure what is called a shutdown effect that improves battery safety in the event of short-circuiting or the like by closing the pores of a separator by causing a resin constituting the separator to melt at a temperature less than or equal to a temperature at which thermal runaway (overheating) occurs in the battery so as to increase the internal resistance of the battery, polyethylene (PE), which has a low melting point, is used as a material for separators.
For the separator, for example, a film that is uniaxially or biaxially drawn so as to increase porosity and improve strength is used. Such a separator is supplied as a film that exists as a single film, and thus is required to have a certain strength in terms of workability. For this reason, it is drawn to secure a certain strength. However, because the crystallinity of such a drawn film increases, and the shutdown temperature also increases to a temperature close to the thermal runaway temperature of the battery, it cannot be said that the margin for securing battery safety is sufficient.
There is also another problem that distortion occurs in the film when drawn, and if the film is exposed to high temperatures, the film shrinks due to residual stress. Because the shrinkage temperature is very close to the melting point, or in other words, the shutdown temperature, in the case of using the polyolefin porous film separator, it is necessary to prevent temperature increase in the battery by reducing the current immediately when the battery temperature reaches the shutdown temperature in the event of a charge failure or the like. If the pores do not close sufficiently, failing to reduce the current immediately, the battery temperature quickly rises to the separator's shrinkage temperature, causing a risk of thermal runaway due to internal short-circuiting.
Various technical developments have been carried out to prevent short-circuiting caused by such separator thermal shrinkage so as to enhance battery reliability. For example, Patent Document 1 proposes an electrochemical device configured of a separator including a porous substrate with a good heat resistance, filler particles, and a resin component for securing the shutdown function.
Also, Patent Document 2 proposes a technique to use, in a lithium ion secondary battery, a separator including a porous film in which secondary particles are bonded by a binder, the secondary particles prepared through sintering or melt-recrystallization of a part of primary particles of a ceramic material having a band gap.    Patent Document 1: WO 2006/62153A    Patent Document 2: JP 2006-147569A
For example, according to the technique disclosed in Patent Document 1, it is possible to provide a safe electrochemical device in which thermal runaway will not easily occur even in the event of overheating.
However, the trend toward higher performance devices to which electrochemical devices, including lithium secondary batteries, are applied is expected to grow continuously in the future, and demand is also expected to increase for, for example, electrochemical devices with improved load characteristics. With this increased demand, demand will rise for development of separators that can enhance load characteristics of electrochemical devices, in addition to further improvement of safety of electrochemical devices and improved reliability of electrochemical devices by suppressing the occurrence of a micro-short circuit due to lithium dendrites, or the like.